Lewis dot notation
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Lewis dot notation
Lewis dot notation is a way of describing and accounting for the outer shell (also called the valence shell) of an atom's electrons and the bonds formed by such electrons. Dots are drawn around the element's symbol to represent the electrons in the valence shell of the atom. Dots shared between atoms represent electrons in a covalent bond. This notation is elegant in its simplicity and utility but has limitations, is sometimes wrong and the justifications for it working are simplistic and potentially misleading.
Use
Each element starts with dots corresponding to the number of electrons in its valence shell. Through the sharing of dots, the next highest noble gas configuration is usually reached for each atom in forming a stable compound.
For elements such as carbon, nitrogen and oxygen for which Lewis dot notation is commonly used a stable molecule is indicated by the presence of eight dots, representing eight electrons, around these atoms. This state is referred to as having a complete octet or noble gas configuration. In this particular case this corresponds filling the 2s and 2p orbitals to have the same configuration as neon. Hydrogen atoms only require two dots (electrons) in a stable molecule, corresponding to a 1s2 configuation (helium). This simplistic correspondance to the periodic table and the atomic orbitals does not always hold such as with Sulfur and Phosphorous which come up in Lewis dot structures often and are sometimes represented with expanded valence (more than eight electrons). This is complicated but is simplistically explained by the use of 3d orbtials. Boron is another exception, although not seen commonly, in that it forms stable compounds without an octet (since it starts with only three valence electrons).
The dots are usually drawn in pairs on the top, bottom, right and left of an atom. Neighboring atoms share the electrons between them. This sharing represent a covalent bond. Multiple bonds are represented by multiple pairs of dots between atoms (which are then missing from other sides). As with triply bonded acetylene:
H:C:::C:H
In this example both carbons have eight neighboring electrons, the hydrogen both have only two, the hydrogens are singly bonded to each carbon and the carbons are bonded to eachother with a triple bond.
External links
- World of Chemistry by Ralph Logan.
- Thinkquest
- Regents Prep run by Oswego City School District links to several tutorials.